This invention relates to a full follow-up type servo-system adapted to attain extremely accurate control of speed and position in a servo-system with no time delay.
In a conventional servo-system, on the output side, is always observed a time delay in response with respect to an inputted speed command or position command, and accordingly, the actual output response is operated differently from the command value. As shown in FIG. 1, for example, the actual response will be represented by a high dimensional curve, i.e. broken line 2 in spite of a tinpezoid-shaped linear acceleration/deceleration command represented by the solid line 1. The deformation of this character in the output response is a significant problem in a case where one object is controlled by a plurality of servo-systems, which will result in a remarkable defect in the controlling performance.
In a numerical control device provided with an ideal electric minor loop (gain "1" up to the frequency ".infin."; no current saturation), the transfer function in an ordinary servo-system is represented by a block diagram of FIG. 2 and also shown in FIG. 3 as a general concept of a block representing a control system 3 and a controlled system 4. Namely, in FIG. 2 .theta..sub.i * (rad/sec) represents a speed command due to a DDA (Digital Differential Analyzer), .theta.* (rad) is a position command and .theta. represents the output position of the controlled system 4. Accordingly, .theta..sub.e (rad) represents the position deviation, which is then inputted into a position loop system (position loop gain .omega..sub.o). The position deviation .theta..sub.e in the position loop system because speed command .theta..sub.o * (rad/sec) and the deviation .theta..sub.e between the speed command .theta..sub.o * and the speed .theta. (rad/sec) is inputted into a speed control system (speed loop gain .omega..sub.c). FIG. 3 shows that a response, i.e. transfer function, to the input command of the output based on the control system 3 and the controlled system 4 is "W" at a time when the speed command and the position command are inputted into the control system 3.
In the servo-system of this type, however, the position deviation .theta..sub.e does not become "0" except for an operation stopping time thereof. That is, when the inputted speed command .theta..sub.i * is constant, a constant position deviation .theta..sub.e is always generated and when the speed command .theta..sub.i * varies, the position deviation .theta..sub.e also varies. For this reason, in a case where a workpiece is cut circularly by means of a machine tool in a controlled system, for example, a working error will be liably caused.